Intel XENIX 286 Programmers Guide (86) - Tenox.tc

XENIX Programming as: Assembler
Based Operands
Based operands are bytes or words in memory with addresses computed by adding a
constant and one of the base registers bp or bx. The operands have the forms
expression ] (bp)
expression ] (bx)
where expression is an absolute expression or symbol that resolves to an absolute.
Based operands are typically used to access structures. The base register points to the
start of the structure, and items in the structure are addressed by an appropriate
expression.
The following examples illustrate based operands:
mov 2(bp), #1000
movb ax, TOP (bx)
neg -4 (bp)
mov ax, (bx) (bp)
Indexed Operands
Indexed operands are bytes or words in memory with addresses computed by adding a
constant and one of the index registers di or si. The operands have the forms
expression (di)
expression (si)
where expression is an absolute expression or symbol that resolves to an absolute.
Indexed operands are often used to access elements in an array. expression points to the
start of the array. The index register is given the index value of the elem ent to be
accessed. Since all array elements are the same length, simple arithmetic on the index
register will select any element.
The following examples illustrate indexed operands:
movb ax,Darray(di)
addb 4096(si), * 1
7-2 1